DN205 - Tiny RGB Video Multiplexer Switches Pixels at 100MHz

Tiny RGB Video Multiplexer Switches Pixels at 100MHz
Design Note 205
Frank Cox and John Wright
Introduction
The LT®1675, a new 3-channel, 2-input 250MHz video
multiplexer, is designed for pixel switching, video graphics and RGB routing. The complete circuit squeezes
into a 16-lead SSOP package and uses only 0.25in2 of
PC board area (Figure 1). The LT1675 features a fixed
gain of 2 for driving double-terminated cables. By
incorporating internal feedback resistors, the circuit
simplifies PC board layout and boosts performance
by eliminating stray capacitance. A single channel 2:1
MUX, the LT1675-1 is available in the small MSOP
package. Table 1 summarizes the major performance
specifications of this new multiplexer, and Figure 2
shows a typical application: switching between two
RGB sources.
Figure 1. Board Photo Actual Size
Table 1. LT1675 Performance, VS = ±5V
PARAMETER
CONDITIONS
TYPICAL
VALUES
–3dB Bandwidth
RL = 150Ω
250MHz
0.1dB Gain Flatness
RL = 150Ω
70MHz
Crosstalk
Between Active Channels
at 10MHz
–60dB
Slew Rate
RL = 150Ω
1100V/μs
Differential Gain
RL = 150Ω
0.07%
Differential Phase
RL = 150Ω
0.05°
Channel Select Time
RL = 150Ω, VIN = 1V
2.5ns
Enable Time
RL = 150Ω
10ns
Output Voltage Swing
RL = 150Ω
±3V
Output Offset Voltage
Supply Current
Supply Current Disabled
05/99/205_conv
20mV
All Three Channels Active
30mA
1μA
RED 1
V+
+1
75Ω
75Ω CABLE
+2
VOUT RED
+1
GREEN 1
75Ω
75Ω
75Ω
+1
BLUE 1
CABLE
+2
VOUT GREEN
75Ω
75Ω
+1
RED 2
75Ω CABLE
VOUT BLUE
+2
75Ω
GREEN 2
75Ω
+1
V–
+1
ENABLE
75Ω
SELECT RGB1/RGB2
BLUE 2
75Ω
DN205 F02
Figure 2. LT1675 Typical Application: Switching Between
Two RGB Sources and Driving Three Cables
Expanding Inputs Does Not Increase
Power Dissipation
In video-routing applications, where the ultimate in
speed is not mandatory as it is in pixel switching, it
is possible to expand the number of MUX inputs by
shorting the LT1675 outputs together and switching
between the two LT1675s with the ENABLE pins. This
technique, shown in Figure 3, does not increase the
power dissipation because LT1675s draw virtually zero
current when disabled.
Add Your Own Logo
The circuit of Figure 4 highlights a section of picture
under control of a synchronous key signal. The technique is used for adding the logo you see in the bottom
corner of commercial television pictures or any type
of overlay signal, such as a crosshair or a reticule. The
key signal has two bits of control, so there can be four
levels of highlighting: unmodified video, video plus
33% white, video plus 66% white and 100% white. Two
LT1675s are configured as a 2-bit DAC, and resistors
on the output set the relative bit weights. The output
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
peak white, which is 0.714V for this NTSC-related RGB
video. The reference white signal is adjusted to a lower
level than peak white to make the effect less intrusive.
of the LT1675, labeled B, is one-half the weight of the
A device. To properly match the 75Ω video cable, the
output resistors are selected so the parallel combination of the two is 75Ω. The output will never exceed
R1
R3
AV = 2
AV = 2
75Ω
75Ω
RED
OUT
75Ω
R2
LT1675 #1
R4
ENABLE
ENABLE
LT1675 #2
DN205 F03
CHIP
SELECT
74HC04
Figure 3. Two LT1675s Make a 4-Input RGB Router
LT1675
RED
V+
+1
75Ω
CABLE
113Ω
+2
VIDEO IN
VOUT RED
+1
GREEN
75Ω
75Ω
CABLE
113Ω
+2
+1
BLUE
VOUT GREEN
75Ω
75Ω
CABLE
113Ω
+1
VOUT BLUE
+2
75Ω
SELECT A
0
0
1
1
SELECT B
0
1
0
1
V–
+1
OUTPUT
NO VIDEO, 100% WHITE
VIDEO PLUS 66% WHITE
VIDEO PLUS 33% WHITE
VIDEO, NO WHITE
SELECT
ENABLE
+1
A
SELECT A
SELECT B
B
LT1675
V+
+1
226Ω
+2
+1
226Ω
+1
+2
+1
+2
226Ω
5V
+1
10k
1.25V
LT1634
10k
V–
SELECT
0.714V
+1
ENABLE
DN205 F04
Figure 4. Logo Inserter
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn205f_conv LT/TP 0599 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1999
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